Fermi Hints at Dark Matter

on April 3, 2014

Using public data from NASA's Fermi Gamma-ray Space Telescope, independent scientists at the Fermi National Accelerator Laboratory, Harvard University, MIT and the University of Chicago have developed new maps showing that the galactic center produces more high-energy gamma rays than can be explained by known sources and that this excess emission is consistent with some forms of dark matter.

No one knows the true nature of dark matter, but WIMPs, or Weakly Interacting Massive Particles, represent a leading class of candidates. Theorists have envisioned a wide range of WIMP types, some of which may either mutually annihilate or produce an intermediate, quickly decaying particle when they collide. Both of these pathways end with the production of gamma rays — the most energetic form of light — at energies within the detection range of Fermi's Large Area Telescope (LAT).

The galactic center teems with gamma-ray sources, from interacting binary systems and isolated pulsars to supernova remnants and particles colliding with interstellar gas. It's also where astronomers expect to find the galaxy's highest density of dark matter, which only affects normal matter and radiation through its gravity. Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.

When the astronomers carefully subtract all known gamma-ray sources from LAT observations of the galactic center, a patch of leftover emission remains. This excess appears most prominent at energies between 1 and 3 billion electron volts (GeV) — roughly a billion times greater than that of visible light — and extends outward at least 5,000 light-years from the galactic center. The researchers find these features difficult to reconcile with other explanations proposed, such as undiscovered pulsars. The gamma-ray spectrum of the excess, its symmetry around the galactic center and its overall brightness, is, however, consistent with annihilations of dark matter particles in the mass range of 31 and 40 GeV.

The scientists note that discoveries in other astronomical objects, such as dwarf galaxies, and experiments on Earth designed to directly detect dark matter particles will be needed to confirm this interpretation.

This animation zooms into an image of the Milky Way, shown in visible light, and superimposes a gamma-ray map of the galactic center from NASA's Fermi. Raw data transitions to a view with all known sources removed, revealing a gamma-ray excess hinting at the presence of dark matter.

Side-by-side maps, with labels. At left is a map of gamma rays with energies between 1 and 3.16 GeV detected in the galactic center by Fermi's LAT; red indicates the greatest number. Prominent pulsars are labeled. Removing all known gamma-ray sources (right) reveals excess emission that may arise from dark matter annihilations.

Side-by-side maps, without labels. At left is a map of gamma rays with energies between 1 and 3.16 GeV detected in the galactic center by Fermi's LAT; red indicates the greatest number. Prominent pulsars are labeled. Removing all known gamma-ray sources (right) reveals excess emission that may arise from dark matter annihilations.

A view of the entire gamma-ray sky from Fermi's LAT instrument, shaded to emphasize the central galaxy. The inset is a map of the galactic center with known sources removed, which reveals the gamma-ray excess (red, green and blue) found there. This excess emission is consistent with annihilations from some hypothesized forms of dark matter.